1. Field of the Invention
This invention relates to a magnet assembly adapted for incorporation as a magnetism generating source in an article, so as to attach such an article to a magnetic substance, or adapted for incorporation as a magnetism generating source in a magnetic lifting device or the like for magnetically retaining a magnetic member or magnetic substance such as steel.
2. Description of the Prior Art
A prior-art magnet assembly as shown in FIG. 1 includes a columnar permanent magnet 10 and a cylindrical casing 12 made of a magnetic material which houses therein the permanent magnet and is open at one end. The permanent magnet 10 is disposed within the casing 12 at a spacing from an inner peripheral surface of the casing coaxially therewith. The permanent magnet 10 is magnetized in the longitudinal direction thereof in a manner to form a pair of magnetic poles at the opposite ends 10a and 10b thereof. One magnetic pole at one end of the permanent magnet 10 contacts the closed end of the casing 12, and a magnetic pole piece 14 contacts the other end of the permanent magnet 10. The magnetic pole piece 14 is attached, through the medium of a ring 16 made of a non-magnetic material, to the inner circumferential edge at the open end of the casing 12.
In the aforesaid magnet assembly, the open end face of the casing 12 and one surface of the magnetic pole piece 14 aligning with the open end face form a magnetic attraction surface 18, on which the magnetic force of the permanent magnet 10 effectively acts as a magnetically attracting force.
The aforesaid magnetically attracting force is proportional to the product of the square of a residual magnetic flux density of the permanent magnet and a cross sectional area perpendicular to a direction of magnetization, namely, a magnetic pole surface. In the structure shown in FIG. 1 in which the permanent magnet is of a columnar shape having magnetic pole surfaces at the opposite ends thereof, when it is desired to increase a magnetic pole surface area, a large permanent magnet is required, thus resulting in a large size magnet assembly. To cope with this, an alnico magnet has been used as a permanent magnet for the magnet assembly which magnet is high in residual magnetic flux density and generates a strong magnetic force from a comparatively small magnetic pole surface.
However, a material for the alnico magnet is an alloy containing a large amount of cobalt or nickel, either of which is expensive, and unstable in demand and supply, suffering from the influence of the international situations. The material is accordingly expensive and hardly obtainable.
An easily obtainable magnet is, for example, a ferrite magnet having a high coercive force. Despite a high coercive force, such a magnet is low in residual magnetic flux density. The residual magnetic flux density of the aforesaid alnico magnet is in the range of 10,000 to 12,000 gauss, and that of the ferrite magnet is in the range of 3,000 to 4,000 gauss. Assuming that the magnetic flux of a magnet wholly acts as a magnetically attracting force, the magnetically attracting force is 4.00 to 5.76 kg/cm.sup.2 in the former and 0.36 to 0.64 kg/cm.sup.2 in the latter. From this, it will be apparent that when it is desired to use a ferrite magnet for the aforesaid prior art magnet assembly, a large permanent magnet is required, which has a magnetic pole surface increased to three or four time that of a conventional alnico magnet, in order to provide for the magnetic attraction surface a magnetically attracting force increased to a level equal to that of the conventional one, thus resulting in an increased sized magnet assembly.